Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas and straight run gasolines. In fact, it is the primary source of octane in the modern refinery. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, such as alumina. The alumina support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and hydrocracking reactions.
Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semi-regenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semiregenerative and cyclic reforming and moving-bed reactors in continuous reforming. In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactivated while the other reactors of the series remain on stream. The "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst, and the catalyst is regenerated in a separate regeneration vessel.
In an era of limited and expensive feedstocks, the demand for additional aromatics (octane) must be satisfied while maximizing both liquid and aromatic yields. For this reason, catalysts offering higher selectivity to liquid products will replace those of lower selectivity. Activity remains an equally important catalytic parameter and must be retained at a level equal to, or ideally greater than, that of current commercial technology. It is recognized in the art that one of the keys to selectivity control is the suppression of cracking reactions occurring over both the metal and acid sites of bifunctional reforming catalysts. Acid cracking reactions lead primarily to propane and isobutane and to higher isoparaffins, which are more difficult to aromatize and hence more prone to both metal and acid site cracking. While a certain level of catalyst acidity is required to initiate certain essential isomerization reactions, overly active acidity leads to both yield loss and deactivation. Specific steps to moderate, or control, acid cracking reactions in catalytic reforming are not common in the art. Elimination of these cracking reactions is desirable for two reasons: first the light C.sub.1 -C.sub.4 gases produced are of lesser value than reformate; and second, activity and liquid yield are tied to the retention and aromatization of feed paraffins.
Sulfur is sometimes used to modify reforming catalyst selectivity by principally suppressing methane formation. While sulfur is effective for this purpose, its use introduces process complications, which could be avoided if catalyst sulfiding were not essential. Furthermore, certain non-reforming active metals have been shown to decrease metal site cracking reactions, principally of the internal type yielding C.sub.3 and C.sub.4 hydrocarbons. Examples of such metals include copper, gold, silver, and tin. Since these metals are normally used in conjunction with sulfur, a universal decline in all light gas products results.
While much work has been done over the years in developing improved reforming catalysts, it has generally focused on such things as concentration and combination of catalytically active metals, and type and pore size distribution of the support material. Other work has resulted in the modification of support materials by introducing a catalytically effective amount of an alkali or alkaline earth metal composite into the support. See for example, U.S. Pat. Nos. 2,602,771; 2,930,763 and 3,714,281.
Although commercially, successful catalysts have resulted from such work, there still exists a need in the art for further improvement, especially with regard to both catalyst activity and selectivity. In accordance with the present invention, a new approach is described wherein reformate yield is improved with simultaneous suppression of both metal and acid site cracking through the use of a novel support material which universally decreases all cracking reactions. While catalyst activity, as measured by conversion, decreases as a consequence of decreased cracking, aromatization activity and aromatics selectivity essential to catalytic reforming are retained and improved. Furthermore, the support materials of the present invention permit catalysts supported on the material to be operated in an unsulfided state.